Many optical proximity sensors known in the art are made using an infrared LED and an infrared light detector. Light from the LED is reflected from an object to be detected back to the detector, and the strength of the received signal is proportional to the distance of the object to be detected from the sensor. Such optical proximity sensors find applications in many portable devices such as mobile telephones, smart phones and PDAs, and can be used, by way of example, to activate or de-activate touch screens.
Examples of optical proximity sensors include the AVAGO TECHNOLOGIES™ APDS-9120 and QPDS-9120 optical proximity sensor packages, which contain an integrated high efficiency infrared emitter and a detector or photodiode housed in a small form factor surface mount device (SMD) package. In the APDS-9120 optical proximity sensor package, as in many other proximity sensor packages manufactured by companies other than AVAGO TECHNOLOGIES™ such as SHARP™, ROHM™ and VISHAY™, an infrared transmitter die must be placed in very close proximity to an infrared receiver die (e.g., mere millimeters apart in the same package).
One significant issue in the design of optical proximity sensors is optical crosstalk, where stray light from the LED falls on the detector and can generate an undesired false signal mimicking a true proximity or object detection signal. Transmitted or received infrared light rays tend to bounce around or reflect internally within such packages, and also to generate scattered, stray reflected or otherwise undesired infrared light rays that bounce off of cover windows or other external objects, resulting in undesired infrared signal crosstalk occurring in respect of both the transmitter and the receiver.
If optical cross talk is too high it can have a detrimental effect on the overall sensing distance the device is capable of achieving, and hence limits device performance. Further exacerbating the foregoing problems is the distinct tendency of infrared radiation to pass substantially or barely unattenuated through many different materials, such as printed circuit boards, many plastics and polymers, and even thin ceramic materials. As the demand for ever smaller proximity sensor packages increases, the problem of eliminating or reducing such infrared signal crosstalk becomes more urgent.
What is needed is an infrared proximity sensor package that features reduced infrared signal crosstalk, but that is inexpensive and easy to manufacture.